For the purpose of this patient, the term mechanism will refer to the logical concept of a machine, which may be realized either as a physical machine, or as a sequence of logical commands executed by a physical machine.
The object of cryptography is to transform information or plaintext into an intermediate form of ciphertext which conceals the information, but which may be used to reproduce the information at a different place or later time. The main problem of cryptography is the construction of intermediate information forms which can be efficiently created and then used, given a special understand or key, but which are extremely difficult to use without that special understanding.
This invention may be used to translate plaintext data into ciphertext using a key, and to recover the original plaintext data using the same key. A particular key value would typically select the generation of a particular random-like or pseudo-random sequence of confusion data; the design of such a generator is well understood and not part of this invention. The confusion data would be combined with the plaintext data in order to encipher or hide the plaintext. The same confusion data would be inversely combined with the ciphertext data in order to decipher or recover the original plaintext. Various other applications are also possible.
One of the ancient forms of cipher is known as substitution. In a monoalphabetic substitution cipher, each plaintext letter has one fixed ciphertext substitute. In this case, the key is the set of substitutions, or substitution alphabet which is used. One way to implement substitution is through the use of a translation table. Conceptually, a translation table is just a list of the output or substitute values, listed in the order of the input or to-be-substituted values. Substitution consists of using the input value to select the particular substitute value or translation which is the result. Each of the possible substitutions in a single table may also be called a substitution element.
All ciphers must, at least in principle, confront an "enemy" or opponent who seeks the information contained in the ciphertext. Substitution is generally easy to solve or penetrate because the different letters in a written language are used with different characteristic frequencies, and substitution does not change those relationships. Accordingly, a cryptanalyst attempting to penetrate the cipher can often make good guesses for various letter substitutions. And, since language is naturally redundant, those guesses generally can be extended with other guesses until the message becomes fully known.
In a polyalphabetic substitution cipher, each letter has multiple fixed substitutes; that is, multiple substitution alphabets are used in some sequence. The different substitution alphabets help to obscure the letter-frequency statistics of the plaintext, but the fixed nature of the substitution alphabets, and their use in a fixed sequence, still allows fairly easy penetration, given a reasonable amount of ciphertext. Although small substitutions are still used, substitution has largely been abandoned as the central part of serious ciphers because it is so easily penetrated.